Ad Removal Tool Makes Ad Bigger

I’ve been putting a lot of work into Law School Discussion lately to fix minor user problems. Last night I noticed that a computer I don’t usually use was showing a huge advertisement on the top of the page. The advertisement pushed all the useable content off the screen (“below the fold” as they say). I kept checking my code and everything looked just fine. I went to sleep frustrated over not knowing why this was happening or how many users might be affected.
It turns out that Norton Internet Security was trying to block a small ad on the top of the page. As the page is downloaded to the user’s computer, Norton takes part of the code out. The problem was that Norton only took the part of the code that controls the shape and size of the ad, leaving a giant default ad in its place.
Until now I haven’t had a strong position on ad removal tools. To the extent that Norton actually modifies my code so that the viewer sees a different version of the page than the one I spent hours perfecting, this software makes me really mad.